illumos gate

The way connection setup works in our SMB client is best explained by looking at:
http://src.illumos.org/source/xref/illumos-gate/usr/src/lib/libsmbfs/smb/ctx.c#1227

A new client (a) asks the driver if we already have a connection to this server,
if not, it (b) makes a door call to the smbiod asking to create one, then (c)
checks in the drive a second time, expecting that the connection now exists.

This issue describes a problem where two threads do the above nearly at
the same time, so both attempt step (b). Only the first caller doing (b)
can succeed. While the first caller is doing the connection setup, any
additional callers that fail in (b) simply because they're not first (which
they know by the EEXIST error) should simply proceed to step (c).

This comment (added to the code) summarizes the fix:
http://src.illumos.org/source/xref/illumos-gate/usr/src/cmd/fs.d/smbclnt/smbiod/smbiod.c#292

+     /*
+     * Create the driver session first, so that any subsequent
+     * requests for the same session will find this one and
+     * wait, the same as when a reconnect is triggered.
+     *
+     * There is still an inherent race here, where two callers
+     * both find no VC in the driver, and both come here trying
+     * to create the VC.  In this case, we want the first one
+     * to actually do the VC setup, and the second to proceed
+     * as if the VC had been found in the driver.  The second
+     * caller gets an EEXIST error from the ioctl in this case,
+     * which we therefore ignore here so that the caller will
+     * go ahead and look again in the driver for the new VC.
+     */